As the miniaturization, the light weight and the high performance of electronic equipment have been successfully pursued in recent years, the miniaturization, the light weight, the high speed signal processing and the implementation at a high density of a printed circuit board are in increasing demand. The printed circuit board technique responds to such a demand by accelerating the formation of a larger number of layers, smaller via holes and finer circuitry. In particular, with the development of the finer circuitry, the gap between lines and via holes is reduced, so that dielectric breakdown can be caused readily due to ion migration. The ion migration is an electrolytic function, referring to a phenomenon in which an electrolyte between metals to which a voltage is applied, the metal at the anode is eluted, moves to the cathode and is precipitated. Since this phenomenon is based on an electrochemical function, it is required that ions flow between the metals. The ion migration in the printed circuit board is generally effected in the following manner: a path of water is generated due to the absorption of moisture at the interface between an impregnated resin and a base material, in foreign fibers attached to a circuit pattern, in cracks in an insulating layer, or the like, and an ion current flows in the path.
A number of methods for preventing the ion migration in the printed circuit board were conventionally proposed, such as a method of making a metal into an alloy in order to suppress the ionization of the metal and a method of adding a reducing agent to the board. Another method of adding a chelating agent to the board in order to capture eluted metal ions was proposed.
However, the aforementioned methods are not necessarily satisfactory as a measure to counter the ion migration in view of the cost and the performance. For example, the cost of producing an alloy is high because expensive palladium is used. Furthermore, for the reducing agent, an aldehyde, hydroquinon, hydrazine or the like is often used. Disadvantageously, they are chemically unstable and are readily degraded by heat or light. Similarly, the chelating agent is likely to be degraded or deactivated by heat.
Furthermore, in addition to the formation of a finer circuitry, the formation of the printed circuit board of a larger number of layers is accelerated. For this reason, more and more boards are formed of an inexpensive and light-weight resin or an inexpensive nonwoven fabric. The resin board generally has a relatively high hygroscopicity, and thus the ion migration is likely to be caused. Furthermore, when a glass or a nonwoven fabric such as aramid is used as the base material, an impregnated resin is unlikely to exist between the base material and a metal foil. This means that a base material fabric not coated with a resin, or a foreign fabric that easily absorbs moisture such as cellulose or the like, is directly in contact with the metal foil. Thus, a path of water is generated due to the absorption of moisture by the board, and then the ion migration is caused. Thus, a printed circuit board having improved resistance to ion migration is significantly desired.